There are two major types out of the many types of interferometers, the Twyman-Green Interferometer and the Fizeau Interferometer. The most popular optical interferometer available on the market is the Fizeau Interferometer. The Fizeau Interferometer is a single optical head measuring instrument. An example of Fizeau Interferometer is depicted in FIG. 1A. Fizeau Interferometer 100 may be used to measure the quality of a flat surface of workpiece 110 facing Fizeau Interferometer 100. Fizeau Interferometer 100 may comprise a single Light Source-Detecting-Computing unit 120 and a single optical head 130.
An example of a Twyman-Green Interferometer 101 is depicted in FIG. 1B. For purposes of explanation, like elements with the Fizeau Interferometer 100 of FIG. 1A are identified with like reference numerals. Twyman-Green Interferometer 101 is distinguishable from Fizeau Interferometer 100 primarily in that a reference flat mirror 135 is used in place of transmission flat 134 as a reference surface.
Light Source-Detecting-Computing unit 120 may comprise laser 121 which may generate light beam 123 through pinhole 122 at a predetermined frequency or range of frequencies. Light beam 123 may pass through beam splitter 124 and pass to optical head 130. Beam splitter 124 is an optical device commercially available which splits the incoming beam into two parts each of about 50% intensity of the original beam, with one beam transmitting through the device and the other reflecting off the device. An example of a beam splitter is a so-called half-silvered mirror.
Optical head 130 may receive light beam 123 from Light Source-Detection-Computing unit 120 and pass such a beam through collimating lens 132 and transmission flat lens 134 to workpiece 110. The side of transmission flat lens 134 facing workpiece 110 acts as a reference surface. Light reflected from workpiece 110 may pass again through transmission flat lens 134 and collimating lens 132 and back to Light Source-Detecting-Computing unit 120.
Beam splitter 124 receives a return beam of light from optical head 130 and passes that beam through lens 125 to detecting device 196. Detecting device 196 may comprise, for example, a charge-coupled device (CCD) camera or the like. Detecting device 196 may output an electrical signal to computer and data processing unit 128.
Computer and data processing unit 128 may, using principles of optical interference, generate data indicating some surface qualities or parameters of the surface of workpiece 110. Such computational techniques are well known in the art, and understanding of such prior art techniques is not necessary for an understanding of the present invention. An example of such an interferometer is discussed, for example, in Ai et al., U.S. Pat. No. 5,321,497, issued Jun. 14, 1994, and Ai, U.S. Pat. No. 5,452,088, issued Sep. 19, 1995, both of which are incorporated herein by reference. Thus, a description of such conventional computational techniques is not provided here.
A double optical head Fizeau Interferometer, although not as popular as the single head model, is also available on the market. As illustrated in FIG. 2, double head Fizeau Interferometer may include one Light Source-Detection-Computing unit 120 (as in FIG. 1A) but with two optical heads 130 and 230. Beam splitter 224 may be suitably modified to simultaneously generate two beams, one each to optical heads 130 and 230, respectively.
The diameters of the output apertures of optical heads 130 and 230 are of different sizes. Such a double-head optical interferometer may be used to measure different types of surfaces selectively. For example, the smaller head 230 may be used to measure spherical or concave surfaces whereas the larger head 130 may be used to measure flat surfaces.
Optical interferometers, such as that illustrated in FIG. 1A, are known for measuring the flatness of various types of flat surfaces. For example, in the computer hard disk drive manufacturing business, optical interferometers are used to measure surface irregularities and "flatness" of individual disks. Interferometers may also be used to compare a non-flat test surface (e.g., concave, spherical, and the like) with a standard surface. Optical windows, mirrors, telescopes, instruments and the like may be measured using optical interferometers.
However, when using a single head optical interferometer (as in FIG. 1A) or even a double head optical interferometer (as in FIG. 2) generally only one surface may be measured at a time. To measure multiple surfaces, each surface must be set up and individually measured. If an optical interferometer can be provided which could measure multiple surfaces, average time for measuring each surface could be reduced.
Moreover, such optical interferometers are rather expensive, as a quality laser and high quality CCD (charge-coupled device) having a high resolution (e.g., 2000.times.2000 pixels @ 12 bits per pixel) are required. If a multiple surface measuring device could be provided, the cost of such components could be shared by a number of individual measuring stations, thus lowering the cost per unit measured.